Compact airborne vehicles, such as unmanned aircraft systems (UAS), hobbycraft, and the like, may operate with full or partial autonomy (e.g., partially controlled by a remote operator) in approved airspaces. However, the airspaces and conditions under which semiautonomous and autonomous UAS may operate are inconsistent from locality to locality or from state to state. To ensure that a UAS may safely operate in, for example, populated areas or areas in which civil or commercial aircraft may be found, the UAS should be able to detect and avoid obstacles, during liftoff and landing as well as inflight. A compact UAS may not have the necessary size, processing power, or complexity for a computer vision based detect-and-avoid system or a phased array antenna-based system, nor may such systems be cost-effective for all UAS operations. Furthermore, it may be desirable for a detect-and-avoid system to “see” independently of the UAS flight plan, e.g., around, behind, or below an airborne position of the UAS. Current radar-based detection systems may be designed for omnidirectional object detection, but strictly in the horizontal plane. Such systems cannot be used, for example, to assist an autonomous landing from an unknown height onto potentially unfamiliar terrain.